U.S. Pat. No. 4,816,642 to Richard L. Dennison, the inventor of the invention presented herein, discloses a temperature control circuit that includes a triac heater control integrated circuit (IC) portion that is commercially available from Motorola, Inc., Phoenix, Ariz. under Motorola's type designation CA 3059 and CA 3079. The CA 3059 and CA 3079 are referred to as zero voltage switches and are similar except that the CA 3059 also has a protection circuit and an inhibit input. The application literature for the CA 3059 and CA 3079 circuits refers to such inhibit input as providing an "external inhibit function." An input of at least 1.2 volts at 10 microamperes is indicted in the application literature as effective "to remove current drive from the thyristor." As noted in U.S. Pat. No. 4,816,642 (supra) the protection circuit of the CA 3059 circuit is subject to several design constraints which precludes its use for a multi-temperature control application.
There is a need for termination of the operation of a temperature control circuit that is controlled by a temperature responsive circuit portion in the event an open circuit occurs in the temperature responsive circuit portion. If such termination protection is absent, an operation of the circuit with such an open circuit could lead to the production of damaging temperatures or create a personnel safety hazard. If the temperature control circuit disclosed in U.S. Pat. No. 4,816,642 (supra), is used for the control of the temperature of a hot melt adhesive applicator, a high uncontrolled temperature is produced if the temperature sensing element of the circuit "opens". It has been the practice to use a thermal fuse in the power line to the heating element of such an applicator which will open when the temperature of the heated block in the applicator reaches approximately 700.degree. F. Such a protection approach protects the operator, but not the applicator. More protection is needed than that which is afforded by hot melt adhesive applicator that employs an adhesive cartridge. The uncontrolled temperature levels created before the thermal fuse opens, when an "open" occurs in the temperature responsive circuit portion, could cause the adhesive cartridge to bunt creating a personnel safety hazard. In cases where the applicator is designed to operate at only one controlled temperature level, the CA 3059 zero voltage switch device could be used to provide the desired protection as the protection circuit in such device is designed to provide protection when the temperature sensor that is used with such a single control temperature application opens or shorts. However, as mentioned above, the protection circuit of the CA 3059 Motorola zero voltage switch can not be used when it is used for multi-temperature applications.
While the application literature for the Motorola device describes the characteristics of the external signal needed to activate the inhibit function that would inhibit operation of a temperature control circuit, the application literature is silent as to how such a signal can be obtained and applied to the inhibit input of the device in the event the temperature sensor for such circuit presents an open. Further, since the Motorola device (CA 3059) has a protection circuit, the use of the inhibit input based for circuit protection upon the occurrence of such an "open" is not even contemplated.